Abstract

Advances in transistors with cut-off frequencies >400GHz have fuelled interest in security, imaging and telecommunications applications operating well above 100GHz. However, further development of passive networks has become vital in developing such systems, as traditional coplanar waveguide (CPW) transmission lines, the most fundamental passive component, exhibit high losses in the millimetre and sub-millimetre wave regime.
This work investigates novel, practical, low loss, transmission lines for frequencies above 100GHz and high-Q passive components composed of these lines. At these frequencies, transmission line losses are primarily due to the influence of the waveguide substrate. We therefore focus on structures which elevate transmission line traces above the substrate using air-bridge technology. Thorough analysis is performed on a range of elevated structures, and analytic / semi-analytic formulae for component figures of merit obtained. These, along with comprehensive 2 and 3D simulations are used to design discrete lines and distributed passive networks, with a focus on the 140-320 GHz frequency range. Innovative fabrication and detailed characterisation of the components are also carried out.
The key result is the development of a novel MMIC compatible transmission line structure, Elevated-Grounded CPW, with a relatively simple fabrication process. EGCPW provides high substrate isolation, resulting in a low losses and high-quality passive networks. 50Ω EGCPW transmission line shows an insertion loss of 2.5dB/mm at 320GHz, 2.5dB/mm less than CPW. EGCPW passive networks, including resonators and filters, show higher performance than both conventional CPW and other forms of elevated CPW. 30-80% improvements in quality factor are shown, and an EGCPW band-pass filter with the centre frequency of 220GHz shows a 12% reduction in bandwidth and 4.5dB reduced in-band loss compared with its CPW counterpart.
Due to the superior performance of MMIC-compatible EGCPW, as well as its ability to support a wide range of characteristic impedances, this structure is suggested as a candidate for widespread use in sub-millimetre wave circuits in order to increase efficiency and reduce losses.